Modis scope trouble

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  • no_common_sense
    Junior Member
    • Jun 2014
    • 4

    Modis scope trouble

    I bought a modis last month off eBay and I'm excited like a kid with a new playstation when the moments come along when I can put it to use. I do have prior experience with an early Modis from a prior job and that experience was mostly on the scan tool side of things. I haven't seen a scope since I got out of the local tech school and it's been a couple years since then so I'm on a learning curve with the machine. But, I've been adapting pretty fast and I enjoy working with the Modis. The moments I can use it are too few and far between so it took me a while to really notice something was wrong.

    Most recently, I was scoping a throttle position sensor on an 01 Oldsmobile with an ecotec when I noticed all my voltages were a half a volt low. If setup is easy enough, I will back probe the power and ground along with the signal circuit and that's how I noticed all the voltages were off. My ground was at a negative half volt, the signal wire was at zero volts, and, of course, the reference voltage was at four and a half volts. Most of my voltage traces can and have been a little choppy on anything I've been scoping. While I was observing my lines on the scope, the traces suddenly all went up to zero, .5 and five volts where they belonged and those choppy lines went perfectly straight and smooth. Unfortunately, I managed to botch saving a snap shot of the event.

    I do have some snap shots saved from a diag on an 04 GMC envoy that's in the ball park of the same behavior of my Modis. There was a throttle position code that I don't remember, But, I scoped two redundant throttle position sensors both before and after to confirm the repair. The problem with the Modis actually turned up in the after repair. My voltage sweep was still within a five volt range, but the low to high signal was from negative two to positive two volts. I made the mistake of thinking that i was looking at the channel one trace with the channel two voltages displayed. Like I said; learning curve. Here's a couple pics for you:



    I've also got this off a 98 S10, 4.3 cam sensor. I replaced the distributor last year and the new distributor was loaded with a new cam sensor. Once I found this turn up, I actually installed the cam sensor out of the old distributor and I was left with pretty much the same waveform. The negative spikes were as deep as -8 volts. I'm more and more suspicious that the Modis is lying to me.


    So what I need to know is: Has anybody experienced this same failure with their Modis and what did it cost you to have it repaired? The money I spent on this box left me short of enough cash to deal with any expensive repairs so I need to start budgeting for a repair someday. Is there a calibration that I'm missing? How do I calibrate the scope if it's possible? Is there just something else I've been doing wrong the whole time? Any ideas?

    Thanks in advance.

    Shaun
  • Witsend
    Banned
    • Nov 2012
    • 2942

    #2
    Erratic scope Behavior

    Before sending out for repair I would try spraying down a clean rag with Static Guard and wipe the unit and probes down . Make sure the Probes all maintain a good clean tight fit at both ends and kept away from other inductive sources , including vibration and shock. In the case of Cam sensor readings like that , I would pull it out and check for a little metallic porcupine on the magnetic tip.

    Comment

    • SPS10.4
      Member
      • Aug 2012
      • 39

      #3
      I've never used the Modis scope so defer to those with experience with the platform. I have used Pico, Vantage Pro, and a few non automotive based scopes.

      Comparing the first and second captures, why are the voltage and timebase settings different? In the first capture, the voltage looks to be a 0-10V range with a timebase measured in seconds. The second capture appears to be +5V-5V with a timebase measured in milliseconds.

      Were both captures set on DC Volts?

      Comment

      • no_common_sense
        Junior Member
        • Jun 2014
        • 4

        #4
        Originally posted by SPS10.4
        Comparing the first and second captures, why are the voltage and timebase settings different? In the first capture, the voltage looks to be a 0-10V range with a timebase measured in seconds. The second capture appears to be +5V-5V with a timebase measured in milliseconds.
        Any time you review a saved pattern on a Modis, it gives you wiggle room to move the zero point of the voltage up and down, but not the time base. I think I changed the time base to compress the pattern a little so I could make a slow sweep of the gas pedal and still see the entire range of movement within the time base on screen. Shopstream lets me magnify the pattern and it's still a dirty signal regardless of how I messed with the time. As far as the voltage goes, I always hit the option in the menu that says "4 channel lab scope". You don't have an option to choose between AC or DC volts at that point. There is still a 10 volt range up the vertical axis on either screen shot, you just gotta consider where the 0 point is. I moved that around to show correlation of the redundant sensors sweeps. The voltage is definitely skewed on the after shot. If you still make regular use of that Vantage Pro, you have just about the exact same scope function as the Modis. At the bottom of your screen in the channel buttons just past the voltage sweep button, is the button that lets you move your zero point on the voltage sweep.

        Originally posted by Witsend
        Before sending out for repair I would try spraying down a clean rag with Static Guard and wipe the unit and probes down . Make sure the Probes all maintain a good clean tight fit at both ends and kept away from other inductive sources , including vibration and shock. In the case of Cam sensor readings like that , I would pull it out and check for a little metallic porcupine on the magnetic tip.
        Thanks for the advice. The connections on the test leads have always felt good and tight. I use Silvertronic pack probe pins with my leads and they have always felt tight too. Considering how pricy they are, they better work. Is Static Guard a particular brand or just a great description of what I need to buy? Would there be any benefit to a very light application of dielectric grease at the connections?

        As far as that cam sensor goes, a thorough cleaning was the first thing I did and I didn't miss a spot inside that distributor and both cam sensors were likewise thoroughly cleaned. It ran perfect for exactly two miles. Summer is here so I should probably just ride my bike anyway.

        Thanks again.
        Last edited by no_common_sense; 06-05-2014, 09:43 PM. Reason: always the details

        Comment

        • Joe Rappa
          Snap-on DSD
          • Aug 2007
          • 2050

          #5
          As already suggested, make sure the scope is not AC coupled. To do that quickly, just connect it to 12V and see what it reads. If it pulses to 12V then returns to 0V, it is AC coupled. Changing it to DC coupling will solve your problem.

          Joe
          "You don't build a reputation on what you're going to do"
          Henry Ford

          Comment

          • no_common_sense
            Junior Member
            • Jun 2014
            • 4

            #6
            I still haven't had time or money to deal with the problems with my Modis and I've been trying to work around them as much as I know better. Even with voltages skewed, I can still get a good enough picture in a lot of cases to find and solve a problem. It actually took me a while to find where the AC coupling option was, and I missed it a few times in plain sight. Once I found it, I checked all four channels and they've never been AC coupled as long as I've owned the machine. It's been lagging and dragging in wierd ways that I figured were relative to the summer's heat so just before I started typing this post, I decided to see if I could at least shoot some compressed air into it. I pulled the left side cover off and noticed a tiny cooling fan in the other end behind the air filter so I grabbed a pocket screw driver to try and turn it. It was stuck but got a little free so I turned the modis back on and still nothing from the fan. Cycled the power one more time to hear the fan start clattering a little and then it started running again. It's still seen better days. It sounds like an electric shaver at the moment. I imagine a lot of problems will go away once that gets replaced.

            As far as that cam sensor waveform goes, I've been dickering around a little when I have some free time and I have eliminated the distributor and the engine wiring harness as the problem. After cleaning the first cam sensor and the tone wheel, I threw a new cam sensor at it to see what would happen. I also threw the original distributor into it and I've gotten the exact same waveform every time. I made some bypass wires from the cam sensor to the PCM so I could try out some new terminal crimpers that have been sitting unused since I bought them. Despite the clean new circuits, I got the same waveform. The bugs started chewing on me so I called it a day and will being checking for voltage drops on the PCM grounds tomorrow.

            I appreciate the help given this far...Thanks.

            Comment

            • Witsend
              Banned
              • Nov 2012
              • 2942

              #7
              I think good clean grounds especially from body to chassis is real important for seeing a clean pattern. For some reason when I use the Power Probe 3 with key on engine not running, it works great, but with engine running it seems sh@tty dirty connection like doesn't like to display voltage, just shows a P for positive and makes raspy sound after the initial beep and sometimes a momentary red led blip and beep probing a hot fuse.It is very sensitive to electrical interference while car is running , maybe induced interference or the alternator ripple effect , maybe defective who knows. Just works so much better if car is not running with the key on. Any one else observe the same behavior with theirs?

              Comment

              • Crusty
                Senior Member
                • Dec 2007
                • 2450

                #8
                Originally posted by Witsend
                I think good clean grounds especially from body to chassis is real important for seeing a clean pattern. For some reason when I use the Power Probe 3 with key on engine not running, it works great, but with engine running it seems sh@tty dirty connection like doesn't like to display voltage, just shows a P for positive and makes raspy sound after the initial beep and sometimes a momentary red led blip and beep probing a hot fuse.It is very sensitive to electrical interference while car is running , maybe induced interference or the alternator ripple effect , maybe defective who knows. Just works so much better if car is not running with the key on. Any one else observe the same behavior with theirs?
                You would probably see less of that on a brand new vehicle, as in less than 6 months old.
                Two things come to play then, batteries are the stabilizer for the entire electrical system and begin to age right away, and the chassis grounds start to corrode within days of the vehicle coming off the assembly line. The frame on an old full size car or the sheet metal of today's unibody vehicles is the other half of the electrical system. Corrosion from piece to piece makes each individual piece of metal become a capacitor that can hold and release electrical charges.
                Some equipment is even sensitive to the static charge of the vehicle. That's why some people would put the grounding straps on their cars designed to drag and dissipate the static.... but that is really old school.... LOL
                Electrical "filtering" can reduce the effect but sometimes too much "filtering" means the equipment doesn't see the quick glitches.

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